The devices in question may, for example, be transistors, for example field-effect transistors, current-rectifying diodes, solar cells, photodetector cells, capacitors, laser diodes, sensor-type devices, memories, or even light-emitting diodes, passive components (inductors, capacitors, resistors), pressure sensors or temperature sensors. It is in particular a question of organic electronic devices on rigid or supple (plastic) substrates. These devices may take the form of independent components or components that are connected together. The stack may be intended to form a single device or a plurality of devices, for example a matrix array of devices.
The invention more particularly applies to the field of the stacks of organic diodes and to the stacks used in organic solar cells or organic photodetectors.
These stacks conventionally comprise a thin, organic, partially organic or inorganic, and insulating, semiconductor or conductive, layer of interest interposed between two layers that are able to conduct electricity.
In the known devices of the prior art, a plurality of types of topological fabrication defects are observed to appear in the layers of interest. These defects are of the free-volume type. They may be through-holes in the layer of interest; they then emerge onto either side of the active layer. It may also be a question of zones of larger free volume between the active layer and a conductive layer than in the rest of the layer of interest. The latter zones are for example blind holes that open onto a single of the conductive layers. It may as a variant be a question of zones in which the active layer has a thickness smaller than a preset nominal thickness in a direction in which the stack is produced.
These zones of larger free volume may be caused by particles on the surface at the moment of deposition of the thin layer, by the presence of substrate defects, such as topological defects or peaks on the surface, or zones having different surface tensions, by the presence of particles in solution (non-dissolved material) when the active layer is deposited by liquid processing, or by local dewetting of the active layer from the deposition surface, etc. These holes may have a lateral dimension of 1 nanometer to several hundred microns. They are generated during the formation of the thin layer.
These defect zones are generated during the fabrication of the device. These zones of larger free volume may generate electrical leaks (zones in which the current will flow more easily between the two electrodes). Parasitic leakage currents may appear in electronic devices in which the thin film of interest is supposed to electrically isolate the conductors forming the conductive electrodes, in particular in the case of organic diodes. Thus, the presence of through-holes in the active layer may lead to the two conductive electrodes to be short-circuited locally.
These parasitic leakage currents are very disadvantageous when they occur in organic photodetectors or current-rectifying diodes. Specifically, in this case, the current in the reverse-bias regime of the diode and in the dark must be very low (of the order of one nA/cm2). Thus, the slightest electrical leakage through defects in the layer of interest may cause this current to increase by several orders of magnitude and drastically and irreversibly degrade the performance of the diode.
These parasitic leakage currents are also disadvantages in organic solar cells, but to a lesser extent. In such a device, the lower the leakage current of the diode the better the solar cell will be able to respond to weak illumination.
Blind holes or zones of larger free volume are also prone to electrical breakdown. These zones are more fragile in the sense that they are liable to degrade more easily under electric field. Now, if in a photodiode the organic layer separating the two electrodes (photoconversion layer) contains at least one defect of sub-thickness type, the electric field increases in the region of sub-thickness and makes it more prone to electrical breakdown. If this electrical breakdown occurs, the electrical current read through the photoconversion layer increases, and the performance of the photodiode is thus degraded.
Thus, solutions for limiting parasitic leakage currents in the active layer of a stack and for limiting the risks of electrical breakdown have already been suggested. A first type of suggested solution aims to limit the number of defects in such active layers. It has in particular been suggested to increase the thickness of the active layer, to filter solutions before their deposition to form the active layer, to use substrates containing few defects, to clean the surface of the substrates before depositing the active layer or to limit the number of particles in the ambient air by working in a cleanroom.
However, the suggested solutions each have drawbacks. Specifically, too great an increase in active-layer thickness tends for example to degrade device performance. Hence, active layers are generally about 1 nm to a few hundred nanometers in thickness. Moreover, filtration requires a solution of good solubility, this not being the case for all the materials currently used for active layers. In addition, filtration steps are difficult to implement on an industrial scale. Substrates containing few defects are substrates having good properties with respect to planarity and are of high cost. With regard to substrate cleaning, it may nonetheless leave certain sizes of particle behind. It is expensive to work in cleanrooms.
It has also been suggested to use double active layers. However, the double-layer concept, which is in particular implemented by wet processing, requires complex techniques in which selective solvents are employed or layers cross-linked to be used in order not to prevent the first active layer from being dissolved during the deposition of the second active layer.
A second type of solution aiming to repair defects in the active layer has also been suggested. It has in particular been suggested, in the international patent application published under publication number WO 2013/182970, to locally etch a first conductive layer on which the active layer is formed through the holes in the organic layer and thus to prevent contact between the first conductive layer and the second conductive layer formed on the active layer. This technique has the drawback of not working with every type of first conductive layer. In particular, this technique does not work when the material of the first conductive layer is difficult to wet etch.
It has also been suggested, in United States patent application US 2012/0126277 to form a first conductive layer, to form an active layer on the first conductive layer then to form an insulating layer on the active layer so that the insulating layer penetrates into a free volume of through-hole type in the active layer. The portions of the insulating layer that are located outside the hole are then removed in order to make a free surface of the active layer appear while leaving the insulating layer inside the hole. A second layer able to conduct electricity is then formed on the active layer and the insulating layer. This process allows parasitic electrical leakage currents through the active layer of a stack of the conductor/active layer/conductor type to be limited, and prevents degradation of the performance of the device produced by means of the stack. In contrast, this method has several drawbacks. The choice of the material forming the insulating layer is limited. Specifically, the latter must be able to fill the hole, be formed on the active layer and be removed from the active layer in order to make the free surface appear without attacking the active layer.